Often it is necessary to cool a working fluid, and it is known for this purpose to use a heat exchanger. Whilst heat exchangers can be of different sizes, the invention is likely to find its greatest utility for the larger sizes of heat exchanger such as those used in power generation plants, land and marine; such plants use a relatively large volume flow and/or speed flow of a liquid coolant.
Heat exchangers usually comprise one or more metallic tubes (typically between 100 and 9000 tubes) suspended between two tube plates, though it is also known to use U-shaped tubes with each tube connected at opposite ends to a single tube plate. Typically, the coolant flows through the tubes, whilst the working fluid passes around and between these tubes and so gives up latent heat (by way of the tubes) to the coolant flowing within the tubes. Each tube may carry external fins (mechanically coupled to or integral with the respective tube) to increase the available surface area for heat transfer, but often the heat exchanger designer will prefer to use the available space to fit a greater number (array) of unfinned tubes, despite the probable cost increase, particularly if the expected operating conditions increase the likelihood of individual tube failure.
Since the working fluid is typically at a higher pressure than that of the coolant, tube failure will result in leakage of the working fluid into the coolant. It is usually uneconomic to treat the coolant to recover the working fluid, and so until the respective tube (when identified) can be replaced it is traditionally taken out of service (with a reduction in heat exchanger capacity). In the interim, escaping working fluid (e.g. oil at 14 kg/sq.cm) may have been discharged with used coolant (e.g. sea water at 2 kg/sq.cm), leading to possible environmental complaints as well as increased heat exchanger operating costs.
Recognised heat exchanger problems are the thinning of a tube wall, particularly adjacent the tube ends, and the pitting or pock-marking of the tube plate(s) around a tube connection position. These problems frequently arise within the tube, or coolant system, in that the coolant can "attack" both at the ends of a tube ("tube" erosion and/or corrosion), and at the tube plate area surrounding the tube ends ("tube to tube plate" erosion and/or corrosion).
Erosion is a common problem in heat exchangers utilising water as a coolant, usually caused by the velocity of flow of the coolant water, especially adjacent the ends of the tube, and over the first few centimeters inside of the tube where the flow may be turbulent. Corrosion can be caused by chemical constituents in the coolant, especially noted for example when the coolant is sea water, used in a heat exchanger for a ship or for a power station located adjacent a tidal estuary.
Thus, these effects singly or together can cause the "as new" tube and tube plate of FIG. 2 to degenerate to the condition of FIG. 3, leading to tube degeneration or failure and leakage of the working fluid into the coolant.
In addition to the erosion shown in FIG. 3, and particularly if the coolant is sea water, there is often localised pitting of the tube plate around the tube end, caused particularly by chemical attack.